U.S. patent number 4,763,032 [Application Number 06/675,144] was granted by the patent office on 1988-08-09 for magnetic rotor bearing.
This patent grant is currently assigned to Fraunhofer-Gesellschaft zur Forderung der angewandten Forschung e.V.. Invention is credited to Gunter Bramm, Pavel Novak.
United States Patent |
4,763,032 |
Bramm , et al. |
August 9, 1988 |
**Please see images for:
( Certificate of Correction ) ** |
Magnetic rotor bearing
Abstract
The present invention provides a magnetic rotor bearing for
suspending a rotor in a contact-free manner, in particular the
rotor of an axial- or radial-centrifugal blood pump, comprising a
permanent- and electromagnet arrangement which stabilizes the
position of, and suspends the rotor in a stator, in particular a
housing, and comprises a circuit arrangement connected to at least
one position sensor or to the position sensor operating circuit
thereof for the rotor. The rotor is suspended in the stator in a
stable, contact-free manner by means of a permanent magnet
arrangement except for a single geometric adjusting degree of
freedom. The position of the rotor is stabilized only within the
geometric adjusting degree of freedom not stabilized by the
permanent magnet arrangement, by a permanent magnet arrangement
located in the rotor and comprising at least one permanent magnet.
The permanent magnet arrangement which co-operates with the
electromagnet arrangement comprises at least one permanent magnet
of the permanent magnet arrangement provided for suspending the
rotor and consists in particular, of the permanent magnets of this
permanent magnet arrangement positioned in the rotor. The circuit
arrangement is a control circuit stabilizing the position of the
rotor, during the absence of external forces, in a position of
minimum energy requirement of the electromagnet arrangement.
Inventors: |
Bramm; Gunter (Munich,
DE), Novak; Pavel (Munich, DE) |
Assignee: |
Fraunhofer-Gesellschaft zur
Forderung der angewandten Forschung e.V. (Munich,
DE)
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Family
ID: |
6215577 |
Appl.
No.: |
06/675,144 |
Filed: |
November 27, 1984 |
Foreign Application Priority Data
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Nov 29, 1983 [DE] |
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3343186 |
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Current U.S.
Class: |
310/90.5;
623/3.14 |
Current CPC
Class: |
F16C
32/0444 (20130101); F16C 32/0429 (20130101); A61M
60/422 (20210101); F16C 2360/44 (20130101); A61M
60/205 (20210101); A61M 60/50 (20210101); A61M
60/82 (20210101); F16C 2316/18 (20130101); A61M
60/148 (20210101) |
Current International
Class: |
A61M
1/10 (20060101); F16C 39/00 (20060101); F16C
39/06 (20060101); F16C 039/06 () |
Field of
Search: |
;308/10 ;128/1D,DIG.3
;623/3 ;417/356 ;340/870.32,870.28,87R ;356/151 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0019313 |
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Feb 1981 |
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EP |
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0060569 |
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Sep 1982 |
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EP |
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1472413 |
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Jan 1969 |
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DE |
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2504631 |
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Aug 1975 |
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DE |
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2342767 |
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Mar 1978 |
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DE |
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2309754 |
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Nov 1976 |
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FR |
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0000987 |
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Nov 1979 |
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IB |
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0003176 |
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Sep 1982 |
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IB |
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2109596 |
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Jun 1983 |
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GB |
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Other References
W Pschyrembol Klinisches Worterbuch, Dr. W. Kraus, Wide Gruyter,
Berlin, w York, 1982, p. 378. .
Textbook of Medical Physiology, A. C. Guyton, M.D., W. B. Saunders
Co., Philadelphia, London, Toronto, 1976, pp. 158-175..
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Primary Examiner: Skudy; R.
Attorney, Agent or Firm: Weingarten, Schurgin, Gagnebin
& Hayes
Claims
We claim:
1. An absolutely-low energy consuming, and relatively-high
electromagnetically efficient, magnetic rotor bearing assembly,
comprising:
a stator;
a rotor having an axis of rotation that is disposed in the
stator;
said rotor has a shaft, wherein the rotor and its shaft are hollow
inside, and have a total density selected to be exactly adapted to
that of human blood;
permanent first magnet means including a permanent magnet mounted
to the rotor cooperative with a permanent magnet mounted to the
stator for magnetically suspending said rotor in said stator by
magnetic repulsion so that the rotor is free from mechanical
contact with the stator and is axially movable by displacement
along the axis of rotation, the permanent magnet mounted to the
rotor is axially staggered with respect to the permanent magnet
mounted to the stator to maximize centering forces;
second magnet means including a permanent magnet mounted to the
rotor cooperative with an electromagnet having a field intensity
mounted to the stator for stabilizing the rotor against
displacement along the axis of rotation; and
means coupled to the second magnet means responsive to a desired
value representing an intended axial position of the rotor and
including a position sensor providing a signal respresentative of
actual axial position of the rotor for providing a field intensity
of the electromagnet of the second magnet means such that the rotor
is always axially moved from its actual position to the intended
axial position in the absence of symmetrical external forces.
2. An absolutely-low energy consuming, and relatively-high
electromagnetically efficient, magnetic rotor bearing assembly,
comprising:
a stator;
a rotor having an axis of rotation that is disposed in the
stator;
permanent first magnet means including a permanent magnet mounted
to the rotor cooperative with a permanent magnet mounted to the
stator for magnetically suspending said rotor in said stator by
magnetic repulsion so that the rotor is free from mechanical
contact with the stator and is axially movable by displacement
along the axis of rotation, the permanent magnet mounted to the
rotor is axially staggered with respect to the permanent magnet
mounted to the stator to maximize centering forces;
second magnet means including a permanent magnet mounted to the
rotor cooperative with an electromagnet having a field intensity
mounted to the stator for stabilizing the rotor against
displacement along the axis of rotation;
means coupled to the second magnet means responsive to a desired
value representing an intended axial position of the rotor and
including a position sensor providing a signal representative of
actual axial position of the rotor for providing a field intensity
of the elctromagnet of the second magnet means such that the rotor
is always axially moved from its actual position to the intended
axial position in the absence of symmetrical external forces;
said position sensor includes at least one pulsed radiation barrier
having a pulsed radiation transmitting element and a cooperative
radiation receiving element; and
further including another pulsed radiation barrier that includes a
pulsed radiation transmitting element and a pulsed radiation
receiving element, said pulsed radiation barriers being mounted to
respective axial ends of the rotor, said pulsed radiation receiving
elements providing respective output signals, a subtraction device
having an input and an output for subtracting the output signals of
the radiation receiving elements of the two pulsed radiation
barriers, an integration device having an input and an output for
integrating the output signal of the subtraction device, a sample
and hold circuit an input of which is connected to the output of
the integration device, a first switch coupled between the output
of the subtraction device and the input of the integration device,
a second switch connected to the integration device for resetting
the integration device, a third switch connected between the output
of the integration device and the input of the sample and hold
circuit, and a control for controlling the duty cycles of the three
switches such that the first switch is always closed when the
radiation transmitting elements are transmitting, the third switch
is closed in each case after the integration of a predetermined
number of output signals from the subtraction device, and the
second switch is closed as soon as the third switch has been
opened.
3. An absolutely-low energy consuming, and relatively-high
electromagnetically efficient, magnetic rotor bearing assembly,
comprising:
a stator;
a rotor having an axis of rotation that is disposed in the
stator;
permanent first magnet means including a permanent magnet mounted
to the rotor cooperative with a permanent magnet mounted to the
stator for magnetically suspending said rotor in said stator by
magnetic repulsion so that the rotor is free from mechanical
contact with the stator and axially movable by displacement along
the axis of rotation, the permanent magnet mounted to the rotor is
axially staggered with respect to the permanent magnet mounted to
the stator to maximize centering forces;
second magnet means including a permanent magnet mounted to the
rotor cooperative with an electromagnet having a field intensity
mounted to the stator for stabilizing the rotor against
displacement along the axis of rotation; and
means coupled to the second magnet means responsive to a desired
value representing an intended axial position of the rotor and
including a position sensor providing a signal representative of
actual axial position of the rotor for providing a field intensity
of the electromagnet of the second magnet means such that the rotor
is always axially moved from its actual position to the intended
axial position in the absence of asymmetrical external forces;
said rotor is adapted to operation as a conveying member in a blood
pump having a suction side, and further including a power control
device, said power control device including a pressure sensor on
the suction side of the blood pump, and a characteristic memory
having its input connected to the pressure sensor, said memory
providing predetermined output signals in response to a
predetermined input signal, and a power control device for
controlling the pumping power in response to the output signal of
the characteristic memory.
4. The assembly of claim 3, wherein the power control device
includes an oscillator having a frequency and an output voltage
coupled to the asynchronous motor of the blood pump, and means for
changing the frequency and output voltage of the oscillator
depending on the load and speed of the blood pump.
5. The assembly of claim 4, further including a second
characteristic memory connected between the asynchronous motor and
the oscillator for changing the operating voltage amplitude of the
asynchronous motor depending on the frequency of the oscillator.
Description
BACKGROUND OF THE INVENTION
This invention relates to a magnetic rotor bearing for suspending a
rotor in a contact-free manner, in particular the rotor of an
axial- or radial centrifugal blood pump, comprising a permanent-
and electromagnet arrangement which stabilizes the position of, and
suspends the rotor in a stator, in particular a housing, and
comprising a circuit arrangement which is connected to at least one
position sensor for the rotor, to adjust the magnetic field of the
electromagnet arrangement.
Although the magnetic rotor bearing according to the present
invention has been developed for the rotor of an axial- or
radial-centrifugal blood pump of an artificial heart, the use
thereof is in no way restricted to such centrifugal blood pumps.
Instead, it is also suitable for a variety of rotors which may be
mounted by means of a contact-free magnetic suspension. Thus, the
magnetic rotor bearing method including the relevent circuit
arrangement connected to one or more position sensors, for
adjusting the magnetic force of the electromagnet arrangement
provided therein is also suitable for mounting and driving
gyroscopes, for example, for the use thereof in space technology,
during flight operations and in submarines or the like, and for
gyro-directional stabilizers which are provided, for example, in
rocket-propelled missiles, and for direct-reading instruments or
the like.
Even with respect to its use in pumps, the magnetic rotor bearing
which is provided according to the present invention is not
restricted to axial- or radial-centrifugal blood pumps, but it may
also be used in other blood pumps which have a rotor which may be
suspended magnetically. Furthermore, the magnetic rotor bearing may
also be used in other rotor pumps. For example, it may be used for
particular advantage in rotor pumps for radioactiveliquids, because
if such rotor pumps are provided with a magnetic rotor bearing
according to the present invention, they are free of friction and
thus of maintenance since they do not require any mechanical
bearings, valves or seals etc. which are subject to wear.
Magnetic rotor bearings for suspending a rotor in a contact-free
manner are known in various embodiments. Thus, U.S. Pat. No.
3,938,913 discloses a flowing device for pumping and/or measuring
the flow of agressive, radioactive or particularly pure flowing
agents, in which apparatus a rotor is suspended in a contact free,
magnetic manner in a housing. Electromagnets are provided in the
housing for suspending the rotor, which magnetic form in each case
a magnetic circuit with magnetic material which is positioned in
the rotor opposite the electromagnets. Only attractive forces are
used in these magnetic circuits by which the rotor is held,
suspended, inside the housing.
However, the use of exclusively attractive forces which are
produced with electromagnetics suffers from considerable
disadvantages:
(a) The stability of the magnetic bearing, i.e., the stability in
the maintenance of the correct suspended position of the rotor, in
which the rotor is positioned at an approximately equal distance in
all radial directions from the inside wall of the housing, is most
unsatisfactory with respect to self- and independently excited
oscillations. A magnetic rotor bearing of this type tends towards
oscillation relatively easily on account of the delayed built-up of
the magnetic field. This delayed magnetic field build-up is a
result of the relatively great inductances which are necessary to
build up the complete magnetic field in each case by electric
currents, and, because of the high permeability of iron (.mu.
relative up to 10,000), high absolute inductances are produced
which result in a delayed increase in the current.
(b) The energy consumption of such a magnetic rotor bearing having
electromagnets is very high, and at the same time the efficiency is
relatively poor, so that undesired thermal energy is produced to a
considerable extent which is not only wasted and the dissipation of
which is not only difficult, but is also extremely dangerous in the
case of blood pumps, because blood albumen coagulates at 42.degree.
C.
(c) The specific forces are relatively low, as are the relative
peak forces, because iron has a high density and fields which are
essentially above 10 kilogauss produce saturation phenomena, .mu.
relative approaching 1.
Furthermore, published European Application No. 0,060,569 or
European Patent Application No. 82 102 188.8, disclose a magnetic
rotor bearing which was previously developed, inter alia, by the
inventors of the present magnetic rotor bearing, for suspending a
rotor of a centrifugal blood pump in a contact-free manner. In that
bearing, a combined electromagnet- and permanent magnetic
arrangement is provided for suspending the rotor, which arrangement
consists of electromagnets which are provided in the stator forming
the housing of the centrifugal blood pump, and consists of
permanent magnets which are positioned in the rotor opposite the
electromagnets, so that they co-operate with the
electromagnets.
This combined electromagnet- and permanent magnet arrangement
basically suffers from the same disadvantages as have been
mentioned above in connection with the magnetic rotor bearing
according to U.S. Pat. No. 3,938,913. The exclusive use of
electromagnets in the stator leads to a relatively high energy
consumption and thus to a considerable generation of heat, and
moreover, a relatively unsatisfactory stability results on account
of the delayed field build-up which is inherent to
electromagnets.
SUMMARY OF THE INVENTION
In contrast thereto, the present invention provides a magnetic
rotor bearing of the initially mentioned type which meets in
particular the following two requirements:
(1) The magnetic rotor bearing should have a high an
electromechanical efficiency as possible, so that the proportion of
energy which is expended for the magnetic suspension and position
stabilization of the rotor and which is converted into heat is as
small as possible in percentage terms and thus the operating
temperature remains restricted to values which are as low as
possible, in particular, to values which lie below the temperature
at which the blood albumen starts to coagulate.
(2) The absolute energy requirement of the magnetic rotor bearing
should be as low as possible, so that the absolute amount of energy
which is converted into thermal energy on account of the magnetic
suspension and stabilization of the rotor, is as low as possible
and thus, moreover, each element of the entire magnetic rotor
bearing is as small as possible, which is important particularly if
the magnetic rotor bearing is to be used in a centrifugal blood
pump which is to be implanted, in particular with respect to the
fact that even a small reduction in the energy requirement of the
centrifigual blood pump itself results in a considerable reduction
in the weight, the volume and the energy requirement of the entire
centrifugal blood pump arrangement which is to be implanted and
which comprises, in addition to the centrifugal blood pump, a
control circuit and, for example, an energy converter or store and
an inductive energy coupling device for coupling energy into the
implanted centrifugal blood pump arrangement.
The objectives are achieved according to the present invention with
a magnetic rotor bearing of the initially mentioned type, in
that:
(a) the rotor is suspended in a stable, contact-free manner in the
stator except for a single geometric adjusting degree of freedom,
by means of a permanent magnetic arrangement which comprises at
least one permanent magnet positioned in the rotor and at least one
permanent magnetic positioned in the stator;
(b) the position of the rotor is stabilized only within the
geometric adjusting degree of freedom which is not stabilized by
the permanent magnetic arrangement, by means of an electromagnetic
arrangement which is provided in the stator and comprises at least
one electromagnet, and by means of a permanent magnet arrangement
which co-operates with the electromagnetic arrangement, which is
located in the rotor and which comprises at least one permanent
magnet;
(c) the permanent magnet arrangement co-operating with the
electromagnet arrangement; at least one permanent magnet of the
permanent magnet arrangement provided for suspending the rotor, and
consists in particular of the permanent magnets of this
permanent/magnetic arrangement positioned in the rotor; and
(d) the circuit arrangement is a control circuit which stabilizes
the position of the rotor, during the absence of external forces,
in a position of minimum energy requirements of the electromagnet
arrangement.
The function according to (d) is performed by a control loop having
a fixed desired value.
This magnetic field bearing according to the present invention has
as high an electromechanical efficiency as possible, because only
the smallest possible proportion of the magnetic forces required
for suspending and stabilizing the rotor is produced
electromagnetically, whereas by far the greatest proportion of the
magnetic forces required is supplied by means of permanent magnets.
In this connection, it is pointed out that a stable suspension of
the rotor by means of permanent magnets is impossible by the
Earnshaw Theorem alone, according to which each mechanical system
which is held balanced in space (3 dimensions) only by means of
permanent magnets is unstable. Moreover, the absolute energy
requirement of the magnetic rotor bearing according to the present
invention is as small as possible, because the control circuit
stabilizes the rotor in a position in which the absolute energy
requirement of the electromagnet arrangement is minimised without
external forces, whereas in order to minimise the energy
requirement during the effect of external forces, a superimposed
regulator which is specified below may be provided.
Thus, the magnetic rotor bearing according to the present invention
is most advantageously suitable for all cases in which the energy
requirement of a magnetic rotor bearing of this type should be
absolutely and relatively low and, moreover, the increase in
temperature which the magnetic rotor bearing experiences due to the
heat which is generated should be as low as possible. Consequently,
the magnetic rotor bearing according to the present invention may
be used particularly advantageously in blood pumps, in particular
axial- and radial centrifugal blood pumps, above all in those cases
in which blood pumps of this type are to be implanted. The
relatively low generation of heat makes it possible to remain
safely below the temperature of 42.degree. C. at which blood
albumen starts to coagulate and which is therefore extremely
dangerous to the human blood circulation.
A particularly preferred embodiment of the magnetic rotor bearing
according to the present invention which is stated above is
distinguished in that:
(1) the permanent magnetic arrangement suspending the rotor
comprises a stationary permanent annular magnet which is positioned
concentrically to the axis of the rotor, which is magnetized in the
axial direction of the rotor, and further comprises a permanent bar
or annular magnet which is axially displaced with respect to the
permanent annular magnet, but is positioned concentrically with
that magnet about the axis of the rotor and is magnetized in the
axial direction of the rotor, and magnetic poles as the same kind
of the permanent annular magnet of the stator and of the permanent
bar or annular magnet of the rotor face one another;
(2) the electromagnet arrangement has an axial electromagnet inside
which the permanent bar or annular magnet of the rotor is
positioned, and
(3) the circuit arrangement is a control circuit which is connected
at its actual value input to the position sensor which determines
the axial position of the rotor.
As a result of this permanent magnet configuration according to the
present invention, maximum centering forces are present in a radial
direction with a given size of the magnet material or of the magnet
arrangement, in particular with a given quantity, a given volume
and a given weight of the magnet material or the magnet
arrangement.
The permanent magnetic arrangement according to the present
invention is preferably constructed as follows:
(1) With a given cross section of the permanent magnets, the length
thereof is to be selected such that the maximum external leakage
flux can develop. To this end, the length of the permanent magnet
must be selected such that the magnetic voltage which is present is
just sufficient for driving the leakage flux through the magnetic
resistance in the leakage field region or volume.
(2) Rare earth magnets, in particular cobalt-samarium magnets are
preferably used as the permanent magnet material. This material is
weakened or demagnetized to the smallest extent under the influence
of a magnetic stray field. Moreover, a high magnetic voltage is
achieved at the shortest magnetic length. Finally, the specific
magnetic energy of the material, based on the weight, is the
greatest of all magnet materials. Finally, the homogeneity of the
magnetization of the material is relatively good.
(3) The external diameter of the permanent bar or annular magnet of
the rotor must lie within the order of magnitude of the internal
diameter of the permanent annular magnet of the stator and must be
slightly smaller than this diameter. Consequently, maximum change,
based on the unit of length of the displacement, of the entire
magnetic field energy of the permanent magnet arrangement takes
place, for example in the case of a radial displacement of the
rotor permanent magnet, with respect to the stator permanent
magnet.
(4) Only extremely homogeneous magnetic material is used, because
otherwise the rotor rotates eccentrically, since the axis of
symmetry of the magnetic field would not otherwise coincide with
the geometrical axis of the magnet.
In the permanent magnet arrangement according to the present
invention, merely the axial degree of freedom requires a
stabilization in order to stabilize the entire magnetic rotor
bearing, and this stabilization is effected by the above-mentioned
electromagnet arrangement which has an axial electromagnet. An
essential feature of the permanent magnetic-electromagnetic mixed
construction according to the present invention is the interaction,
which takes place in this case, between the permanent magnetic
field and the electromagnetic field, by which the axial degree of
freedom is stabilized by the electromagnet in that the field
intensity of this electromagnet is controlled by a control loop
which comprises a position sensor device determining the position
of the axis of the rotor in an axial direction, and by which a
signal is produced which indicates the actual position of the rotor
axis in the axial direction and is compared in the control loop
with a desired value indicating the desired position of the rotor
axis in the axial direction, whereupon the control circuit of this
control loop regulates the electrical supply to the electromagnet
such that the axial position of the rotor axis is brought into the
desired position and is held therein.
A particular rapid reset of the rotor axis into the desired
position when the rotor axis is moved out of this desired position
by any forces, is achieved in that, in an embodiment of this
invention, the control circuit has an output amplifier which
controls in a current proportional manner. Consequently, the
current in the exciting coil of the electromagnet may be adjusted
within a very short time to a specific current value necessary for
returning the rotor axis to its desired axial position. As a result
of this, a delay in the field build-up by the inductance of the
electromagnet may be eliminated.
The above-mentioned control with a fixed axial desired position of
the rotor is indeed optimum for short-term external adjusting
forces acting on the rotor, with respect to the energy consumption,
because such short-term forces disappear again according to
definition after a relatively very short time, namely after a few
1,000th, 100th or 10ths of a second. However, if external axial
adjusting forces act on the rotor axis in the long term, i.e. from
a few tenths of a second to a few hours, a substantially increased
energy consumption may be produced for the stabilization of the
rotor axis in the above-mentioned desired position. In order to
reduce this increased energy consumption, a particularly preferred
embodiment of the magnetic rotor bearing is designed according to
the present invention such that the control circuit has a
superimposed regulator which moves the motor position by the
control circuit during the long-term influence of substantial
external forces, directly or indirectly by changing the actual
value of the position sensor, and which reduces, preferably
minimizes, the energy consumption of the magnetic rotor bearing.
Experiments on practical embodiments of the magnetic rotor bearing
according to this invention have shown that the energy requirement
of this magnetic rotor bearing amounts to about, for example 0.1
Watt for the position regulation of the rotor axis in the absence
of substantial external forces, and this energy requirement may
increase up to, for example 10 to 15 Watts during the effect of
substantial external forces, i.e. increases by 100 to 150 times if
the above-mentioned superimposed regulator is not provided.
Substantial external forces are, in particular, unilaterally acting
acceleration forces, of the type which arise, for example in motor
vehicles, trains and aeroplanes or the like during starting up and
braking, as well as acceleration due to gravity as long as it
becomes very asymmetric with respect to the axial direction of the
rotor, which may happen, for example if a patient who hse,
implanted, an artificial heart with a rotor which is mounted by
means of the magnetic rotor bearing according to the present
invention, sleeps on his side or the like. When it is considered
that in the case of such an implanted artificial heart, the power
which is required has to be coupled inductively into the body of
the patient, it is most essential for the power required for
stabilizing the position of the rotor to amount to only about 0.1
Watt instead of, for example from 10 to 15 Watts, as mentioned
above.
The superimposed regulator preferably comprises the following
devices in particular:
(a) a detection device detecting the presence of external
forces;
(b) an adjusting device moving the rotor out of its stabilized
centre position into an eccentric stabilization position; and
(c) a comparison device comparing the energy requirement of the
rotor bearing in the stabilized centre positions with its energy
requirement in the eccentric stabilization position.
A superimposed regulator which is constructed in this manner
therefore operates if the detection device detects substantial
external forces, such that this deivce causes the adjusting device
to move the rotor out of the previous stabilized centre position
some way into an eccentric stabilization position, so that
permanent-magnetic forces of the bearing magnets which arise in an
asymmetric position of the rotor counteract the external
forces.
Thereupon, the comparison device compares the previous energy
requirement with the new energy requirement of the magnetic rotor
bearing, and depending on whether the new energy requirement is
higher or lower than the previous energy requirement, the adjusting
device moves the rotor again in a following step in the sense of a
reduction of the energy requirement of the magnetic rotor bearing,
and this gradual or continuous adjustment is preferably effected
until a new stabilization position of the rotor axis is achieved in
which the magnetic rotor bearing has a minimum energy requirement
under the respective substantial external force.
By means of a corresponding response delay, it is possible for the
superimposed regulator not to respond to short-term external
forces, for example, such forces which last for only fractions of
seconds.
Although the most varied kind of detection devices are suitable for
a superimposed regulator of this type, a current meter for
measuring the electric current flowing through the electromagnet
arrangement is preferably used as the detection device, or a
currnet meter for measuring the complete electric current which
flows through the control loop comprising the position sensor or
sensors, the control circuit and the elecromagnet arrangement.
A preferred embodiment of the adjusting device is a position
sensor-desired value changing device, in which case this desired
value, which corresponds to the desired axial position of the rotor
axis, may either be directly changed in that the desired value fed
into the control loop is changed accordingly, or indirectly in that
the desired value fed into the control circuit is maintained
constant, but an additional quantity is added to the actual value
released by the respective position sensor or an additional
quantity is subtracted from this actual value.
Finally, the comparison device preferably comprises a current meter
for measuring the electric current flowing through the
electromagnet arrangement or through the complete control loop, and
a sample- and hold circuit which memorizes and compares the
measured values of the current. A delay member may possibly also be
used for this purpose as long as the value is held for a sufficient
period.
The most varied position sensors which are known according to the
prior art may, in principle, be used as position sensors, as long
as the position information is present only in the stator and as
long as a sensor part which may be in the rotor, does not require
energy or an electrical contact. However, magnetic field sensors
cannot generally be used (magnet in the rotor), because such
sensors may be influenced by the magnetic field of the magnetic
rotor bearing and consequently unfavourable reactions result.
Capacitive sensors for determining position are unusable in
practice or are very problematic for different reasons, and that
is, firstly, because an electric field is thereby produced in the
blood and a problematic contacting is necessary, if the capacitor
plates of a sensor of this type are provided between the stator and
the rotor, and secondly, because in those cases in which the two
capacitor plates of the sensor are provided on the housing, a
change in the dielectric takes place between these capacitor plates
due to the rotor. According to the present invention, radiation
sensors are preferably used as position sensors, in particular such
radiation sensors which operate with light radiation or sound
radiation, preferably infrared sensors or ultrasonic sensors.
In particular, the magnetic rotor bearing according to this
invention is preferably constructed with respect to its position
sensor arrangement such that the position sensor comprises at least
one pulse radiation nbarrier which is operated by the rotor shaft
and has a pulse-controlled radiation transmitting element and a
radiation receiving element.
The position sensor device which preferably has two position
sensors and a position sensor operating circuit connected thereto
may, in particular, comprise the following:
(1) two pulse radiation barriers, each of which co-operates with an
axial end of the rotor shaft;
(2) a subtraction device subtracting the output signals of the
radiation receiving elements of the two pulse radiation
barriers;
(3) an integration device integrating the output signals of the
subtraction device;
(4) a sample- and hold circuit connected to the output of the
integration device; and
(5) a delivery control device for controlling the closing and
opening stroke of a first, second and third switch, of which the
first switch is provided between the output of the subtraction
device and the input of the integration device, while the second
switch is a switch causing the reset of the integration device, and
the third switch is positioned between the output of the
integration device and the input of the sample-and hold circuit,
and the delivery control device controls the closing times of the
three switches such that the first switch is always closed when the
radiation transmitting elements are transmitting, whereas the third
switch is closed in each case after the integration of a
predetermined number of output signals from the subtraction device,
and the second switch is closed each time the third switch is
opened.
As already emerges from the statements made above about the sensors
preferably used, the pulse radiation barriers are preferably either
infrared light barriers or ultrasonic barriers. The advantage of
infrared sensors or infrared light barriers and of ultrasonic
sensor or ultrasonic barriers resides in particular in the fact
that both types of these sensors or barriers are decoupled from the
electric or magnetic properties of the surroundings, because
infrared light or ultrasonic radiation is virtually unaffected by
these electric or magnetic properties. Apart from this, the LED
diodes used during the employment of infrared light have a very low
energy requirement, are low in price and small.
In connection with the magnetic rotor bearing described above or
irrespective thereof, an asynchronous motor may be provided as the
drive of the rotor, which motor is connected to an oscillator as
the operating voltage source, the frequency and output voltage
amplitude of which may be changed depending on the load and speed,
such that the torque released by the asynchronous motor correlates
with the moment of the load at the respective speed of the
synchronous motor, in particular, the respective moment of the load
of the rotor on the motor characteristic line of the asynchronous
motor lies slightly below the breakdown torque and above the
breakdown speed.
An electronically commutated direct current drive is also
particularly advantageous. To this end, permanent magnets, in
particular, in the form of magnetic discs or other magnetized areas
in the rotor, may preferably be provided around the outer periphery
of the rotor or inside its outer periphery, as alternating magnetic
poles, which are opposite wire coils in the stator which are
bilaterally or unilaterally iron free and through which current
flows and which detect as far as possible the total magnetic flux
of the above-mentioned permanent magnets of the rotor, the electric
current in the wire coils being commutated such that the rotor is
made to rotate.
If the rotor is used as a conveying member in a blood pump, in
particular a centrifugal blood pump, a power control device may be
provided which comprises the following:
(a) a pressure sensor on the suction side of the blood pump;
(b) a characteristic memory which is connected by its input to the
pressure sensor and assigns and releases a specific output quantity
to a predetermined input quantity; and
(c) a power control device controlling the actual blood pumping
power corresponding to the output quantity of the characteristic
memory, preferably an oscillator, the frequency and output voltage
of which may be changed depending on the load and speed and which
is connected to an asynchronous motor which drives the blood pump,
as the operating voltage source of this asynchronous motor.
Although a magnetic rotor, of the type provided by the present
invention, is preferably driven in the manner described above, as
an alternative, the drive mechanism may also be designed in any
other suitable manner, for example, as a synchronous motor, an
electronically commutated electromotor which is excited by
permanent magnets, etc.,. Furthermore, although the above-described
asynchronous motor drive is preferably used in the case of a rotor
which has a magnetic rotor bearing designed according to the
present invention, this asynchronous motor drive may also
advantageously be used in the case of differently mounted rotors,
for example, in such rotors which are mounted magnetically only by
means of electromagnets, or in rotors which have a mixed
electromagnetic-permanent magnetic rotor bearing of the type
described in U.S. Pat. No. 3,938,913 and in Published European
Application No. 0,060,569, or in the case of any other magnetic,
mechanical or hydraulic rotor bearings etc.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention, as well as other advantages and features
thereof will now be described in more detail in the following with
reference to FIGS. 1 to 5 of the drawings using a few particularly
preferred embodiments of the rotor bearing according to the present
invention and using a preferred embodiment of a rotor drive of this
invention.
FIG. 1 is an overall view of a particularly preferred embodiment of
a magnetic rotor bearing according to the present invention and of
a rotor drive of this invention, of a rotor of a double-flow
radial-centrifugal blood pump, shown in a perspective view,
together with the permanent and electromagnets in which the rotor
of this centrifugal blood pump is mounted or driven in a magnetic
contact-free manner, while the control circuit of the magnetic
rotor bearing and the drive circuit for the rotor are shown in a
block view;
FIG. 2 is a sectional view through a magnet arrangement for the
magnetic bearing of a rotor of a blood pump according to FIG. 1,
the motor being indicated in dashed lines;
FIG. 3 shows a preferred embodiment of a proportional control
output stage which is provided in the control circuit of the
magnetic rotor bearing according to FIG. 1;
FIGS. 4a and 4b each show curves which indicate the path of the
voltage or the current with respect to time which may be achieved
on the winding of the electromagnet which is connected to the
output stage shown in FIG. 3;
FIG. 5 shows a curve which illustrates an example of the optimum
choice for the operating parameters of the asynchronus motor which
is used in FIG. 1 as the rotor drive, and
FIG. 6 shows an embodiment of an axial-centrifugal blood pump
according to the present invention.
DETAILED DESCRIPTION OF THE INVENTION
The magnetic rotor bearing and the rotor drive shown in FIG. 1 are
used in this case for mounting and driving the rotor 1 of a
double-flow radial-centrifugal blood pump 2 which has two blood
in-flow channels 3a and 3b and one blood outlet 4 formed in a
stator 5 serving as the housing of the centrifugal blood pump 2,
the rotor 1 being mounted in the stator 5 in a magnetic,
contact-free manner.
The magnetic rotor bearing comprises a first and a second
permanent- and electromagnet arrangements each generally indicated
by 6a and 6b respectively, each surrounding one of the two axial
end regions of the rotor 1, and a control circuit 7 for operating
the electromagnets of the permanent- and electromagnet arrangements
6a and 6b. Moreover, the magnetic rotor bearing comprises two
position sensors, designed in each case as pulse radiation barriers
8a and 8b for determination of the axial position of the rotor 1,
and a position sensor operating circuit 9 which receives input
signals from the pulse radiation barriers 8a and 8b, and releases a
corresponding output signal to the control circuit 7.
The rotor drive comprises two driving electromagnets 10a and 10b,
which, together with the metallic body of the rotor 1 form as
asynchronous motor, and comprises a power control device 11 having
a pressure sensor 12 positioned on the suction side of the
centrifugal blood pump 2, here, in the blood in-flow channel 3b.
The rotation of the rotor 1 is indicated by arrows 13, and the
blood flow by arrows 14.
The rotor 1 together with its shaft which may consist of, for
example polished aluminium, is hollow inside, so that its total
density is exactly adapted to that of the blood surrounding it. The
flow of blood is guided through the two blood in-flow channels 3a
and 3b from both sides to the disc-shaped rotor 1 and is driven by
the rotor 1 centrifugally via an annular blood channel 15 and
centrifugally out of the stator 5 serving as the housing, so that
consequently no asymmetric forces arise on the rotor 1 due to the
operation thereof.
In one embodiment given by way of example, the energy requirement
of the rotor drive is from about 8 to 10 Watts under a constant
blood pressur of 0.133 bars, and is thus substantially lower than
that of blood pumps which operate in a pulsating manner. In this
embodiment, the energy requirement of the magnetic rotor bearing is
only about 0.5 Watts. On account of this low energy requirement,
the sets of batteries or accumulators for the energy supply may be
relatively small and light and may be implanted, and only have to
be renewed after one or two days, or, for example, recharged at
night.
In particular, the permanent- and electromagnet arrangements 6a and
6b of the magnetic rotor bearings which are shown in FIG. 2 on an
enlarged scale as compared with FIG. 1, are designed such that the
rotor 1 is suspended in a stable, contact-free manner in the stator
5 FIG. 1 in the region of each of its two axial ends by means of a
rotor and stator permanent magnet arrangement in such a way as to
provide single geometric adjusting degree of freedom which extends,
in this case, in the axial direction of the rotor 1. This permanent
magnet arrangement which suspends the rotor 1 comprises a permanent
annular magnet 17a or 17b which is positioned concentric to the
axis 16 of the rotor and is stationary in the stator 5, and
comprises a permanent bar magnet 18a or 18b (FIGS. 1 and 2) or a
permanent annular magnet 28c or 18d (FIG. 6) which is positioned
concentric with the permanent annular magnet 17a, 17b about the
axis 16 of the rotor and inside the rotor 1.
In particular, the permanent annular magnet 17a or 17b is
magnetized in the axial direction of the rotor 1, as indicated in
FIGS. 2 and 6 by the north pole N and the south pole s.
Furthermore, the permanent bar magnet 18a or 18b or the permanent
annular manget 18c or 18d FIG. 6 of the rotor is staggered axially
with respect to the allocated permanent annular magnet 17a or 17b
of the stator such that it is positioned just outside the permanent
annular magnet 17a or 17b of the stator or just toughes that
magnet. In this manner, an optimum radially centering force effect
is produced by the permanent magnet arrangement consisting in each
case of a permanent annular magnet 17a or 17b in the stator and a
permanent bar magnet 18a or 18b or a permanent annular magnet 18c
or 18d in the rotor. The permanent bar magnets 18a and 18b or the
permanent annular magents 18c and 18 d are also magnetized in the
axial direction of the rotor (see the details concerning the poles
in FIGS. 2 and 6), and the rotate together with the rotor 1 about
the rotor axis 16, or may be mounted so that they are freely
rotatable in the rotor 1 and such that they do not then rotate with
the rotor.
Moreover, each of the two permanent- and electromagnet magnet
arrangements 6a and 6b have an annular electromagnet 19a or 19b
inside which the permanent bar magnet or the permanent annular
magnet of the rotor is positioned. The electromagnets 19a and 19b
are provided concentric to the rotor axis 16 in a stationary manner
in the stator 5.
Consequently, the electromagnet 19a or 19b interacts magnetically
with the allocated cylindrical permanent bar magnet 18a or 18b or
with the permanent annular magnet 18c or 18d of the stator 5, the
permanent bar magnet 18a or 18b or the permanent annular magnet 18c
or 18d of the stator 5 for its part interacting magnetically with
the allocated permanent annular magnet 17a or 17b of the stator.
The axial position of the rotor 1 is centered and stabilized by the
electromagnets 19a and 19b interacting with the permanent magnets
18a and 18b or 18c and 18d. In symmetry and in the central
position, the axial permanent magnetic forces are compensated
precisely so that at stable equilibrium is produced.
In order to effect the centering and stabilization, the magnetic
field of the two electromagnets 19a and 19b which are connected to
the output 21 of the control circuit 7 via corresponding lines 20a
or 20b is constantly regulated.
FIG. 6 shows an embodiment of an axial-centrifugal blood pump in
which the same reference numerals have been used for those parts
which are similar to those in FIGS. 1 & 2. Only the features
which differ from those of FIGS. 1 and 2 will be described in the
following, but reference will also be made to the description of
FIGS. 1 and 2.
Unlike the radial-centrifugal pump described above, the rotor 1 in
FIG. 6 is of a tubular design and bladeor screw profiles 100 are
provided on the inner periphery of the tube for conveying the blood
through the inside thereof, these profiles running to a point at
the blood inlet at 100a and at the blood outlet 100b. In this case,
instead of the permanent bar magnets 18a and 18b which are provided
in FIGS. 1 and 2, permanent annular magnets 18c and 18d which have
already been described, are provided in the tube outer casing of
the rotor 1 at both axial ends, the same type of polarization
(facing oppsoite poles) as in FIGS. 1 and 2 being used (see the
poles N and S in FIG. 6).
A backflow of blood against the direction of arrows 14 should
preferably be prevented or reduced in the gap 101 between the outer
periphery of the rotor 1 and the inner periphery of the stator 5.
This may be effected by conveyor blades (not shown) which are
provided on the outer periphery of the rotor 1 or in openings which
are formed between the permanent annular magnet 18c and/or 18d and
a short circuit ring 10c of the rotor 1 and pass through the
tubular outer surface of the rotor 1, or by designing these
openings as conveying channels extending in screw shape in a radial
and axial direction, which convey the blood in the direction of the
arrows 14 in the gap 101.
Moreover, stationary deflection paddles 102, 103 may be provided in
the blood in-flow channel 3c upstream of the rotor 1 and/or in the
blood outlet 4 downstream of the rotor 1, to achieve a satisfactory
efficiency.
To drive the axial-centrifugal blood pump of FIG. 6, a single
electromagnet 10 is positioned in the stator 5 instead of the two
electromagnets 10a and 10b of FIG. 1, which electromagnet 10
co-operates as an asynchronous motor with the short circuit ring
10c of the rotor 1 in the same manner as explained with reference
to FIGS. 1 and 5. Reference numeral 105 indicates the exciter
winding of the electromagnet 10.
The pulse radiation barriers 8a and 8b co-operate through bores
104a and 104b in the permanent annular magnet 17a or 17b with the
pointed axial ends 23a and 23b of the rotor 1.
The stabilizing and centering control of the axial rotor position
in the centrifugal blood pump according to FIGS. 1 and 6 will now
be described in more detail.
For this control, the actual value input 22 of the control circuit
7 is connected to the output of the position sensor operating
circuit 9 at which a signal is received which represents the actual
position of the rotor 1 as sensed by means of the pulse radiation
barriers 8a and 8b in co-operation with the axial ends 23a and 23b
of the rotor 1.
For the operation thereof, radiation transmitting elements 24a and
24b, for examples an LED diode operating within the infrared range
or an ultrasonic transmitter, are coupled via a line 25 to the
output of an amplifier 26, the input of which is connected to the
output of an oscillator 27. The amplifier 26 and the oscillator 27
belong to the position sensor operating circuit 9, the signal
inputs 28a and 28b of which are each connected to one of the
radiation receiving elements 29a or 29b of the pulse radiation
barriers 8a or 8b. These radiation receiving elements 29a and 29b
are preferably an infrared sensor or an ultrasonic sensor.
The control circuit 7, which receives its actual value from the
position sensor operating circuit 9 or, in the simplest case, from
a position sensor determining the axial position of the rotor 1,
controls the electric current intensity in the electromagnets 19a
and 19b so that the actual value received at the actual value input
22 finally conforms with a predetermined desired value for the
axial position of the rotor 1.
To this end, the control circuit 7 comprises, for example, a
pre-amplifier 30 directly connected to the actual value input 22,
the output of which amplifier is connected to a subsequent servo
amplifier 31 which controls an output amplifier 32 which regulates
in a manner proportional to the current. Moreover, the control
circuit 7 has a hierarchical or superimposed regulator 33 which is
described in more detail in the following.
The servo amplifier 31 is preferably a PDT servo-amplifier, i.e., a
proportional-differential servo-amplifier with a time function
element. The advantages of this servo-amplifier are, in particular,
that system noise is suppressed and the regulating stability is
improved.
The output amplifier 32 converts a given input voltage into a
proportional output current, and that is, independently of the
electrical load conditions. In this manner, almost instantaneously,
i.e. for example step-shaped increases in current may be achieved
in spite of the relatively great inductance of the regulated
electromagnets 19a and 19b, in that the voltage, as shown in FIG.
4a is initially increased to a substantially greater extent than
corresponds to the desired increase in current, and then reduced
relatively quickly to a voltage value which actually corresponds to
the desired current.
An example of an output amplifier 32 which regulates in a manner
proportional to the current is shown in FIG. 3. This output
amplifier 32 comprises an operational amplifier 34 the output of
which is earthed via the magnetic field coil 35 provided as the
load, of the electromagnet 19a or 19b and a low-value resistor R.
The voltage drop at the resistor R which is proportional to the
current through the magnetic field coil 35 is returned via a
voltage divider formed by resistors R.sub.1 and R.sub.2, to the
negative input 36 of the operational amplifier 34 which may also be
termed the negative feedback input. The positive input 37 of the
operational amplifier 34 is coupled to the output of the servo
amplifier 31.
The construction and operation of the pulse radiation barriers 8a
and 8b and of the position sensor operating circuit 9 will now be
described.
As already mentioned, the pulse radiation barrier 8a co-operates
with one axial end 23a of the rotor 1, so that depending on the
axial position of the rotor, radiation from the radiation
transmitting element 24a, as indicated by the two small arrows, may
arrive at the allocated radiation receiving element 29a or is
hindered by the axial end 23a of the rotor 1 from falling on the
radiation receiving element 29a. Accordingly, the radiation
transmitting element 24b and the radiation receiving element 29b
co-operates with the other axial end 23b of the rotor. In the
present case, infrared radiation transmitting elements are provided
as radiation transmitting elements and infrared radiation receiving
elements are provided as radiation receiving elements.
The oscillator 27 which may also be, for example quite generally a
pulse generator, preferably has a frequency in the range of from 10
to 100 kHz, amounting more preferably to 40 kHz. The infrared
radiation transmitting elements are selected so that they produce
infrared radiation in a wavelength region in which blood has a low
absorbtivity, i.e., about 1 .mu.m.
It is pointed out with respect to the above-mentioned frequency
range of the oscillator 27, that the systme would react too slowly
if the pulse frequency was selected to be substantially lower than
10 kHz, and that is, on account of the signal processing in the
integrator which is described later on. On the other hand, if the
pulse frequency of the oscillator 27 is selected to be
substantially higher than 100 kHz, the electro-optical efficiency
of the light-emitting diodes used as infrared radiation
transmitting elements is unfavorable and the evaluation is
complicated by high-frequency effects.
The infrared radiation receiving elements are preferably
photodiodes or phototransistors, the maximum spectral sensitivity
of which lies in the same range as the maximum emitting power of
the infrared radiation transmitting elements.
The part of the position sensor operating circuit 9 by which the
output signals of the radiation receiving elements 29a and 29b are
processed into the actual value to be supplied to the input 22 of
the control circuit 7, comprises a subtraction device 38 which
subtracts the signals emitted by the two radiation receiving
elements 29a and 29b from each other, and a subsequently connected
integration device 39. A sample- and hold circuit 40 is connected
downstream of this integration device 39. Moreover, a first switch
S1 is provided between the output of the subtraction device 38 and
the input of the integration device 39. A second switch S2 is
provided between the reset connection 41 of the integration device
39 and earth and is used for resetting the integration device 39 in
each case. Finally, a third switch S3 is positioned between the
output of the integration device 39 and the input of the sample-
and hold circuit 40, the output of which is coupled to the actual
value input 22 of the control circuit. To control the closing and
opening of the switches S1, S2 and S3, a delivery control device 42
is provided in the position sensor operating circuit 9 which
receives input signals from the oscillator 27 synchronously with
the signals which arrive at the signal inputs 28a and 28b because
the switches S1, S2 and S3 are opened and closed in a manner which
is described in more detail later on.
So that the position sensor operating circuit 9 does not simply
integrate with the electrical impulses released by the radiation
receiving elements 29a and/or 29b and based on the radiation
pulses, with a high time constant, which would imply a response
delay in the entire control of the electromagnets 19a and 19b and
thus could possibly lead to natural oscillations or to the position
of the rotor no longer being guaranteed through the effect of rapid
external forces, the construction and operation of the position
sensor operating circuit 9 is as will now be described in detail in
the following.
First of all, the subtraction device 38 which has already been
mentioned is connected between the radiation receiving elements 29a
and 29b which, when infrared radiation is used, are for example
phototransistors or photodiodes or other photoreceivers, and the
integration device 39 which simultaneously operates as a
pre-amplifier. This subtraction device 38 subtracts the output
signals of the radiation receiving elements 29a and 29b, and has,
in particular, the following purpose and the following
advantages:
(a) Any disturbance by foreign radiation is reduced, and that is,
in particular because possible foreign radiation which comes from
outside has effect to approximately the same or to a similar
intensity at both radiation receiving elements 29a and 29b and
during the subtraction of the signals of these radiation receiving
elements, the proportion of the foreign radiation is mutually
cancelled out completely or to a substantial extent. Moreover, the
likelihood of foreign radiation of precisely 40 kHz and in proper
phse disturbing the transmitted signal is very small.
(b) The working range of the complete sensor arrangement is
expanded, as regards determining the axial position of the rotor 1,
and that is, in connection with the arrangement of the pulse
radiation barriers 8a and 8b, such that these pulse radiation
barriers overlap only in a small working range, i.e. the transition
regions between the complete closing condition and the complete
opening condition of these two pulse radiation barriers 8a and 8b,
which do not completely coincide, but overlap in a small area only
with respect to the complete transition region in each case, which
means that the two pulse radiation barriers 8a and 8b are
simultaneously located in this their transition region only over a
short length of their transition region.
(c) As a result of the subtraction of the output signals of the two
radiation receiving elements 29a and 29b, the characteristic line
of the position sensor arrangement, i.e., of the complete
arrangement of the pulse radiation barriers 8a and 8b and the
position sensor operating circuit 9 is linearised in the
overlapping working region which is defined above.
The integration device 39 operates such that it integrates the
difference pulses obtained at the output of the subtraction device
38 which are obtained by the subtraction of the pulse-shaped output
signals of the radiation receiving elements 29a and 29b, in a
periodic manner only via, in each case, a few individual difference
pulses, for example four differences pulses, and then passes this
integration value on to the sample-and hold circuit 40 at its
output via the switch S3. the integration may be an amplitude
integration which is independent of the width of the difference
pulses. The integration time constant may be selected to be very
high, i.e., it may tend towards infinity.
The following advantages in particular are obtained by this type of
further processing of the difference pulses in the integration
device 39:
(1) In spite of the very high integration time constants of the
integration device 39, the entire position sensor arrangement
reacts almost immediately, i.e., after a few individual difference
pulses, for example, after four difference pulses, to a change in
amplitude, so that the position sensor arrangement responds very
quickly to axial changes in position of the rotor 1, because the
pulse repetition frequency lies simultaneously in the range of from
10 to 100 kHz, preferably at 40 kHz, as mentioned above. In the
case of the above-mentioned preferred pulse repetition frequency,
the response time therefore amounts to about 10 msec. This response
time may be extended or shortened as required, depending on the
practical requirements, by changing the pulse repetition frequency
and/or the number of periodically integrated individual difference
pulses, and thus may be adjusted in an optimum manner.
(2) Since no relatively great changes in the axial position of the
rotor 1 are usually produced, conditioned alone by the inertia of
the rotor 1 within the above-mentioned short response time of the
entire position sensor arrangement, the change in the actual value
which appears at the actual value input 22 is virtually the same as
the integrated value, and thus the regulation of the electromagnets
19a and 19b by the control circuit 7 to a new value of the
integrated amplitude of the difference pulses do not take place
abruptly, but gradually on account of the method of integration
which is used, so that no actual value jumps take place by which
natural oscillations could be excited.
In the sample- and hold circuit 40, the integration value obtained
at the output of the integration device 39 is firmly retained in
each case and passed on to the actual value input 22 as an actual
value until the sample- and hold circuit 40 is supplied with a new
integration value by the integration device 39 via the switch
S3.
The delivery control device 42 opens and closes the switches S1, S2
and S3 in the following manner, to which end it receives its clock
frequency via a line 43 from the pulse generator or oscillator
27.
(a) The first switch S1 is always closed when the radiation
transmitting elements 24a and 24b are connection and thus the
radiation receiving element 29a and/or 29b releases an output
signal, i.e., a difference pulse is generated at the output of the
subtraction device 38. However, in the interval between the
appearance of successive difference pulses, the switch S1 is
opened. This means that the difference pulses of the subtraction
device are passed on to the integration device 39 synchronously
with the pulse operation of the pulse radiation barriers 8a and 8b.
In this manner, the influence of possible disturbing radiation is
substantially reduced.
(b) The third switch S3 is closed after, in each case, a
predetermined number of difference pulses, for example, after in
each case four difference pulses, so that each of the integration
values which are received periodically by the integration device
39, is passed on to the sample- and hold circuit 40 and thus is
made available as an actual value for the control circuit.
(c) After the respective integration value has been passed on to
the sample- and hold circuit 40 by closing the switch S3 and the
switch S3 has been reopened, the second switch S2 is closed in
order to cancel the old integration value in the integration device
39.
(d) The second switch S2 and the third switch S3 are preferably
closed in each case only within an opening period of the first
switch S1. However, the switches S2 an S3 may also be closed for a
longer period, but they may never be closed at the same time, but
the switch S3 must always be closed before the switch S2, and the
switch S2 must only be closed after the switch S3 has been opened,
so that a zero integration value is not released to the sample- and
hold circuit 40 and thus a completely false and, moreover, an
irregularly changed actual value, is not received at the actual
value input 22. As long as the switch S2 is closed while observing
these conditions, no further integration takes place, so that, due
to a longer closing period of the switch S2, the response time of
the complete position sensor arrangement to axial changes in
position of the rotor 1 may be prolonged. If the switch S3 is kept
closed for a longer period while observing the above-mentioned
conditions, the number of the periodically amplitude-integrated
difference pulses is consequently increased, i.e., the response
time of the complete position sensor arrangement is also
prolonged.
The delivery control device 42 is preferably a digital control
device, particularly because it is used to operate the switches S1,
S2 and S3 which in each case have only two conditions, namely an
open and a closed condition.
The general principle on which the position sensor operating
circuit 9 described above is based consequently resides in the fact
that the position sensors, in the present case the pulse radiation
barriers 8a and 8b, which are used for determining the axial
position of the rotor 1 are operated in terms of pulses,
disturbances are eliminated by forming the difference of the
position sensor signals, the difference pulses are integrated
intermittently over in each case only a few individual difference
pulses, the difference pulses to be integrated in the integration
device are delivered synchronously with the appearance of the
radiation pulses in the position sensors, and the integration value
which is determined intermittently is also held at the output of
the position sensor operating circuit 9 and thus at the actual
value input 22 of the control circuit 7 in the intervening periods
between two integration intervals.
A possible influence of disturbing radiation on the position
sensors and thus on the actual value of the control circuit is
eliminated as a whole in particular by the following measures:
(1) by pairing narrow-band radiation transmitting elements 24a and
24b with narrow-band radiation receiving elements 29a and 29b, for
example by pairing narrow-band infrared radiation transmitting
elements and infrared radiation receiving elements, the main
radiation transmitting- or receiving elements preferably using
infrared radiation which lies at a wavelength of about 1 .mu.m;
(2) by a capacitive coupling which has been mentioned above, with
low time constants between the radiation receiving elements 29a and
29b on the one hand and the subtraction device 38 on the other
hand, i.e., by a differential element which only allows through
rapid voltage changes;
(3) by the subtraction device 38, as described above in in more
detail;
(4) by a synchronised take-over of the difference pulses from the
subtraction device 38 into the integration device by means of a
suitable delivery control, as described above; and
(5) by the periodic integration of, in each case, only several
individual difference pulses, for example four difference pulses;
consequently, even the effect of possible, very short-term
disturbances, the duration of which is of the order of magnitude of
the duration of an individual difference pulse or shorter, is
eliminated, because while a portion caused by such a short-time
disturbance may be considerable with respect to a single difference
pulse, it may be reduced by amplitude integration of several
different pulses.
The construction and operating method of the superimposed regulator
33 will now be described.
The superimposed regulator 33 comprises a detection device 44 which
detects the presence of external forces, and also comprises a
adjusting device 45 which moves the rotor 1 out of its centre
position which is stabilized in the axial direction, into an
axially eccentric stabilization position during the effect of
external forces, and comprises a comparison device 46 which
compares the energy requirement of the rotor bearing in the rotor 1
located in an axial direction in the stabilized centre position
with the energy requirement of the rotor bearing when the rotor 1
is located in an axial direction in an eccentric stabilization
position.
In particular, the detection device 44 may be, for example, a
current meter which measures the electric current flowing through
the electromagnets 19a and 19b. Consequently, in the view of FIG.
1, the detection device is connected to the output of the output
amplifier 32 or to the electromagnets 19a and 19b via lines 47a and
47b. Alternately, the detection device 44 may also be a current
meter which measures the electric current flowing through the
entire control loop which comprises the position sensor
arrangement, the remaining part of the control circuit and the
electromagnet arrangement. A further alternative is for the
detection device 44 to comprise an acceleration sensor which
detects the axial orientation or, generally, the direction of a
respective external force.
In the present case, the detection device 44 is connected to the
adjusting device 45 via the comparison device 46, because it
detects the external forces by determining the energy requirement
of the rotor bearing and thus not only determines the energy
requirements of the rotor bearing in the axially stabilized centre
position of the rotor, but also the energy requirement of the
eccentric axial stabilization position of the rotor, and delivers
the quantities to the comparison device 46 which compares them. If,
on the other hand, the detection device 44 directly determines the
external forces, for example by an acceleration sensor, without
determining the energy requirement of the rotor bearing, it may be
directly connected to the adjusting device 45, in which case
another detection device is provided for determining the respective
energy requirement or a quantity proportional to this energy
requirement and delivers to the comparison device 46 the energy
requirement values of the rotor bearing to be compared.
In the latter case, the adjusting device 45 is a control device
which responds to the output signal of the detection device 44 and
to the output signal of the comparison device 46 and which releases
a signal, for example, a voltage at its output, by which the
desired value for the axial position of the rotor 1 is modified via
a line 48 which leads to the pre-amplifier 30 or to the
servo-amplifier 31. The desired value is modified either directly
or indirectly in that a quantity modifying the actual value is
added to, or subtracted from the actual value obtained at the
actual value input 22, which corresponds to a desired value
displacement. In the circuitry according to FIG. 1, the adjusting
device 45 is only connected to the output of the comparison device
46, because the detection device 44 does not simply determine the
presence of external forces, but rather the energy requirement of
the rotor bearing, so that an increase in this energy requirement
is established by the comparison device and a corresponding output
signal is released by the latter to the adjusting device which
causes the adjusting device 45 to move the rotor axially. In any
case, the axial movement of the rotor takes place by an increase or
a reduction in the current flowing through the electromagnets 19a
and 19b as a result of the desired value modification.
The comparison device 46 may comprise, for example a sample- and
hold circuit which compares the energy- or power requirement values
of the electromagnets 19a and 19b and of the entire control loop
which, in the present case, are determined by the detection device
44, in different axial positions of the rotor 1, and, based on this
comparison, releases an output signal to the adjusting device 46
which causes the device 45 to shift the axial position of the rotor
1 in the sensor of lower energy-or power requirement values of the
electromagnets 19a and 19b. To this end, the comparison device 46
has a memory which stores the previous energy- or power
requirement, so that it may be compared with a new energy- or power
requirement. In this manner, the axial position of the rotor 1 may
be adjusted gradually by successive energy- and power requirement
comparisons and by successive modifications of the desired value
for the axial rotor position by in each case, small amounts, until
the energy- or power requirement has reached its minimum.
The purpose of the superimposed regulator 33 is to vary the desired
value of the rotor position along the degree of freedom stabilized
by the electromagnetis, i.e., in the present case, in an axial
direction, during the long-term effect of external forces, so that
the energy consumption of the position control or of the magnetic
position stabilization of the rotor 1 is reduced, preferably
minimized.
As described above in the general part of the description, this
energy consumption may consequently be reduced by a factor of about
100 to 150.
The general method by which this is effected and which is realized
in the arrangement according to FIG. 1 is summarized in the
following:
(a) the presence of external forces is detected;
(b) the rotor is moved from its stabilized centre position into a
new stabilization position, and
(c) it is established whether the new stabilization position
results in a lower energy requirement for the magnetic bearing of
the rotor; this lower energy requirement results if the new
stabilization position is such that the permanent magnets which are
provided in addition to the electromagnets for the magnetic rotor
bearing produce a counter-force to the detected external force;
(d) the stabilization position of the rotor is changed until a
reduced, preferably minimized energy requirement for the rotor
bearing is established.
In the above-mentioned step (b), instead of the rotor being moved
in the first step with a restricted step width, in an arbitrary
manner in any direction out of its stabilized centre position, it
may also be moved in the first step in the sense of a reduction of
the energy requirement, as long as not only the presence of the
external force, but also the direction thereof (positive or
negative) in the direction of the degree of freedom which is
stabilized by the electromagnets is established by the detection
device. If, moveover, the detection device also determines the
magnitude of the external force which appears in the direction of
the above-mentioned degree of freedom, a gradual change in the
stabilization position of the rotor may be omitted, and the rotor
may be moved in one step into its new stabilization position
corresponding to a minimum energy requirement of the magnetic rotor
bearing, while it is programmed according to the direction and
magnitude of the force component which acts along the stabilized
degree of freedom.
The rotary drive of the rotor 1 will now be described in more
detail with reference to the upper part of FIG. 1 and with
reference to FIGS. 5 and 6.
This rotary drive may be designed in a variety of ways, for example
as an eddy current drive, in particular by means of an asynchronous
motor, as a synchronous motor, as an electrically commutated
electromotor or the like. An asynchronous motor is preferably
provided as the drive of the rotor 1 and it comprises the annular
driving electromagnets 10a, 10b (FIG. 1) or 10 (FIG. 6) in the
stator 5, and the rotor 1 as a squirrel-cage rotor (FIG. 1) or a
squirrel-cage rotor ring short circuit 10c (FIG. 6) and the
three-phase driving electromagnets 10a, 10b or the driving
electromagnet 10, the windings of which function as field
excitation coils. These windings which are not shown in FIG. 1 and
are indicated in FIG. 6 by reference numeral 105 are preferably
connected in a star-connection with a phase shift of
.rho.=60.degree.. The potential gradients on 49a 49b and 49c which
lead to the windings of the driving magnets 10a, 10b or 10, follow
the expressions given in FIG. 1 at these supply lines, where A
represents the maximum voltage amplitude, f represent the frequency
of the alternating voltage and .rho. represents the above-mentioned
phase displacement of the three phases 1, 2 and 3 on the supply
lines 49a, 49b and 49c with respect to one another.
An oscillator 50 is provided as the operating voltage source, to
which the supply lines 49a, 49b and 49c are connected via lines
51a, 51b and 51c and a respective power amplifier 52a, 52b and 52c
whose amplification level may be controlled and which amplifies the
alternating voltage supplied by the oscillator 50. The degree of
amplification, and thus the output amplitude A of the alternating
voltage received at the output of the power amplifiers 52a, 52b and
52c is controlled by means of a control voltage which is supplied
via line 53 to amplification level-control inputs. The frequency of
the oscillator 50 may also be changed and may be adjusted via a
corresponding control voltage which is supplied to the
frequency-control input 55 of the oscillator 50. In this manner,
the frequency f and the output voltage amplitude A of the
oscillator on the windings of the driving electromagnets 10a and
10b or 10 may be changed.
This change in the frequency and the output voltage amplitude takes
place in a pre-determined or programmed manner by means of a first
characteristic memory 56, the output of which is coupled to the
frequency-control input 55 of the oscillator 50, and by means of a
second characteristic memory 57, the output of which is coupled to
the amplification level-control inputs 54a, 54b and 54c of the
power amplifiers. The input of the second characteristic memory 57
is connected to an output of the oscillator 50 via a
frequency-to-voltage converter 58. In this manner, the amplitude A
is changed depending on the respective operating frequency f which
the oscillator 50 produces according to a predetermined
characteristic which is indicated symbolically on the
characteristic memory 57.
To control the frequency f of the oscillator 50 with the first
characteristic memory 56, a control signal is given to the input 59
of the latter which may be produced in a variety of ways. In the
present case of the control or regulation of the drive of a blood
pump, namely the centrifugal blood pump 2, this control signal is
produced by pressure sensor 12 and is delivered via a line 60. This
pressure sensor produces an output voltage representing the
pressure on the venous side or on the suction side of the blood
pump, as the control signal. This output voltage is converted by
the first characteristic memory 56 according to a Frank-Starling
characteristic into a corresponding control voltage for the
oscillator 50, such that the pump characteristic of the centrifugal
pump 2 acting as an artificial heart corresponds to the
physiological conditions, as specified by Frank and Starling via a
relation usually known as Starlings law as specified for example in
the medical dictionary of Pschyrembel, published by Walter De
Gruyter, Berlin and New York, 1972.
To this end, the characteristic memory 56 clearly allocates a
specific control voltage 55 at its output to a predetermined
control signal at its input 59, the memory 56 passing this control
voltage 55 on to the frequency-control input of the oscillator 50,
as mentioned.
The second characteristic memory 57 has a characteristic such that
the operating voltages obtained on the supply lines 49a, 49b and
49c are varied in their frequency f and their output voltage
amplitude A depending on the load and speed, such that the torque
provided by the asynchronous motor, i.e. in the present case the
torque produced at the rotor 1, correlates with the moment of load
in the case of the respective speed of the asynchronous motor, i.e.
in the present case, of the rotor 1.
In a particularly favourable and preferred embodiement which will
now be described using the simple drawing of FIG. 5, the respective
moment of load of the rotor on the motor characteristic line I or
II of the asynchronous motor lies just below the breakdown torque
and above the breakdown speed. FIG. 5 shows two selected motor
characteristic line I and II which apply in each case to an output
voltage amplitude A.sub.1 or A.sub.2 and an allocated frequency
f.sub.1 or f.sub.2, and that is, the motor characteristic line
represents in a conventional manner the torque M depending on the
speed N (for example in revolutions per minute) of the asynchronous
motor.
The points P.sub.1 and P.sub.2 on the operating characteristic line
B of the motor obtained lie, according to the above-mentioned
definition of this selected operating line, in each case just below
the breakdown torque M.sub.1 or M.sub.2 and above the breakdown
speed N.sub.1 or N.sub.2 of the motor characteristic line I or II
valid for the relevant output voltage amplitude A.sub.1 or A.sub.2
and for the relevant frequency f.sub.1 and f.sub.2. This applies to
all other points on the operating characteristic line B of the
asynchronous motor.
In this manner, the rotor 1 of the centrifugal blood pump 2 is
driven at a minimum expenditure of energy and at the same time, the
actual blood pumping power corresponds to the Frank-Starling
characteristic, i.e., is adapted optimally to the physiological
conditions of the human blood circulation. The characteristic
memories 56 and 57 provided for this purpose may be, for example a
resistor network matrix or an electronic memory, considering the
fact that the respective characteristic is non-linear.
The magnetic rotor bearing according to the present invention has
in particular, the not exclusively, the following advantages:
(a) it manages to operate at the lowest energy requirement.
(b) it remains operative in particular where it is provided in
moving parts, devices or the like, for example rockets, in spite of
the movement of the respective device, in particular despite the
acceleration or delay thereof, so that it may be used without
restriction in all cases in which the stator as such undergoes a
movement.
(c) It may also be widely used in particular on account of the
advantages specified above under (a) and (b), where it has to
operate with a non-recurring energy supply which cannot be
supplemented during operation, i.e. in so-called insular
operation.
* * * * *